Completely porous microspheres for chromatographic uses

ABSTRACT

This invention relates to an improved packing material for chromatographic columns, prepared from a powder of uniform-sized porous microspheres composed of a plurality of interconnected colloidal oxide particles.

United States Patent 1191 Kirkland 1 Jan. 1, 1974 COMPLETELY POROUSMICROSPHERES [56] References Cited FOR CHROMATOGRAPHIC USES UNITEDSTATES PATENTS [75] lnventor: Jos J. Kirkland, Wilmington, 3,535,26510/1970 Baron et al. 210/198 C D v 3,505,785 4/1970 Kirkland 55/67 [73]Asslgnee: s at f i gg gs Primary Examin John Adee P g Attorney-WilkiThomas, Jr. [22] Filed: Apr. 7, 1972 211 App]. No.: 242,038 [57]ABSTRACT U.S. Cl 55/67, 55/386, 210/198 C In B0ld 15/08 F1 fSearch55/67, 74 55/387, 389; 210/31 0, 198 c,

' invention relates to an improved packing material hromatographiccolumns, prepared from a powzed der of uniform-s1 porous microspherescomposed of a plurality of interconnected colloidal oxide particles.

16 Claims, 5 Drawing Figures PAIENIEDJM 1 I974 SHEET 1 If 3 FIG. I

Pmmemm 1 14 3,782,075

SHEET 3 OF 3 F I 6 4 v OH 1 H3C CH3 IMPURITIES T IN STANDARDS 16 0.0|Asm L N ff=825 eff/f=- OH TIME, Seconds FIG-5 TIME, Seconds COMPLETELYPOROUS MICROSPHERES FOR CHROMATOGRAPHIC USES BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to animprovement in chromatography and chromatographic columns. Morespecifically, it relates to a novel packing material for chromatographiccolumns which comprises a powder of uniform-sized, completely porousmicrospheres.

2. Discussion of the Prior Art One type of packing used inchromatographic applications consists of a powder of completely solidparticles. Such a support has the disadvantage of providing a minimumsurface .area for the processes involved.

Completely porous bodies of controlled porosity, such as those disclosedin British Patent 1,171,651 for Porous Silica Grains which issued to M.LePage et al, have also been used as chromatographic supports, but theseparticles suffer from the fact that they incorporate, as an integralpart of the particle, non-siliceous impurities consisting of ions ofalkali snd alkali earth metals, such as Na". Bodies containing suchimpurities have a non-homogeneous surface composition, which may cause asolute in the carrier phase to be preferentially adsorbed at some sitesof the surface relative to others.

Completely porous particles without such impurities have been made byspray drying a sol of colloidal silica, as disclosed in US. Pat. No.3,301,635, which issued to H. E. Bergna et al on Jan. 31, 1967. Thisspray drying technique, however, produces particles of non-uniform sizewhich are generally larger than 20 microns. Both of these factorsdetract from the usefulness of such particles in chromatographicapplications.

The primary difficulty with those completely porous particles producedby spray drying techniques is that they are non-uniform in size. Becauseof the difference in time required for the mobile phase to diffuse intoand out of particles of varying size, the use of a powder of non-uniformparticles is equivalent to the use of a non-homogeneous support medium.Furthermore, the incrase in resolution in modern chromatographictechniques has to a large extent been dependent on an increase in theefficiency of column packing. Packing efficiency is decreased when theparticles are of nonuniform size. While it is possible to produce apowder with a large disparity in particle size and then separate theparticles into uniform sizes, no convenient way to do this has beenfound when the particles are below 20 microns in size.

The other difficulty with spray dried products is the size of theparticles formed. Large particles, with pores lying deep within theparticle, create deep pools of stagnant mobile phase which result inband spreading and loss of resolution in the chromatographic instrument.This difficulty has been reduced by using superficially porous particlessuch as those disclosed in US. Pat. No. 3,505,785 which issued to .I. J.Kirkland on Apr. 14, 1970. In these superficially porous particles, thecore of the particle is a solid sphere which is surrounded by severalmonolayers of smaller silica particles. An increased surface area isprovided by the small outer particles and the stagnant pools of mobilephase are reduced by virtue of the fact that the central core of theparticles is impervious. The use of a solid central core, however, is aninefficient method of removing the problems caused by deep pores becauseit requires an increase in the size of the particles to achieve an incrase in the surface area.

It is an object of the present invention to provide a powder, for use asa chromatographic packing material, which is optimized for the type ofseparation desired. The desirability of using small particles in liquidchromatography has been discussed by various authors including L. R.Snyder in the Journal of Chromatographic Science 7, 352 (1969) and .l.H. Knox in the same journal at page 614. Furthermore, the desirabilityof using small porous particles has been discussed by authors such as.I. .I. Kirkland in Analytical Chemistry 43, 36A (1971). Nowhere,however, has there been disclosed or suggested a chromatographic packingmaterial, particularly liquid chromatographic packing material with anideal combination of properties for the type of separation desiredachieved by using uniform size particles which are not only small,spherical and porous, but which also have a controlled pore size andgenerally a relatively large surface area. It is the object of thisinvention to provide such a packing material.

SUMMARY OF THE INVENTION According to this invention, there is provideda chromatographic packing material comprising a powder of uniform-sizedporous microspheres having an average diameter of about 0.5 to about 20microns, preferably 1.0 to 10 microns, substantially all of which have adiameter ranging from about 0.5 to about 1.5, preferably 0.8 to 1.2,times the average diameter of the microspheres in the powder. Themicrospheres themselves consist essentially of a plurality ofuniform-sized colloidal particles having a refractory metal oxidesurface arranged in a interconnected three-dimensional lattice defininga plurality of internal pores having uniform and controlled dimension.The size of the pores is controlled by the size of the colloidalparticles used to form the microspheres and the surface area of themicrosphere is controlled by the amount of sintering used to impartstrength to the particles. In the preferred embodiment, the specificsurface area of said microparticles is only slightly less than thespecific surface area of the colloidal particles from which they areformed, and the colloidal particles occupy less than 50% of the totalvolume of the microparticle, with the remaining voltime being occupiedby the interconnected pores. The colloidal particles can be composed ofmaterials selected from the group consisting of silica, alumina,zirconia, titania, ferric oxide, antimony oxide, tin oxide orcombinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS The invention can better be understoodand explained by reference to the following figures:

FIG. 1 shows a single porous powder particle useful as thechromatographic packing of the present invention. The microsphere isgenerally indicated by the number 10, the colloidal ultimate particlesby the number 11 and the pores in the microspheres by the number 12.

FIG. 2 is a diagrammatic view of a section of a microparticle showingthe lightly coalesced colloidal particles ll separated by uniform-sizedpores 12.

FIG. 3 shows a high efficiency separation of a mixture of hydroxylatedaromatic compounds using liquidliquid chromatography and one of thechromatographic columns of the present invention.

FIG. 4 shows a high efficiency separation of a mixture of hydroxylatedaromatic compounds using liquidsolid chromatography and one of thechromatographic columns of the present invention.

FIG. 5 shows a high efficiency separation of three polystyrene fractionsusing exclusion chromatography and one of the chromatographic columns ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION A single microsphere whichcomprises the powder used in the chromatographic column of the presentinvention is shown diagrammatically in FIG. 1. A portion of thismicrosphere is shown diagrammatically in FIG. 2. The microspheres arecomposed of a plurality of colloidal particles 11 which areinterconnected in a threedimensional matrix which occupies less than 50%of the volume of the microsphere. The remainder of the microsphere iscomposed of uniform-sized pores 12.

The microparticles used in the powder are characterized by the fact thatthey are spherical, and have an average diameter of about 0.1 to aboutmicrons, preferably about 1.0 to about 10 microns. Furthermore, they areuniform in size which means that less than 5% of the particles in thepowder have a diameter less than about 0.5 times the average diameter ofthe microspheres in the powder, and less than 5% have a diameter greaterthan about 1.5 times the average diameter. Preferably this range isabout 0.8 to about 1.2 times the average diameter. Finally themicrospheres have controlled pore dimensions and a relatively largesurface area and pore volume. By control of the sintering used toprovide strength to the particle, the specific surface area of themicrospheres can be as high as about 90% of the specific surface area ofthe colloidal particles from which they are made and the microsphereswill still have a strength sufficient to allow them to be used withoutfracturing. The size of the pores contained in the microparticles willdepend primarily on the size of the colloidal particles used to producethe microspheres.

The average diameter of the pores in the microspheres of the presentinvention, at a pore diameter of 1,000 A, is about half the calculateddiameter of the ultimate particles making up the microsphere. Thisdiameter is calculated from the equation D 6000/dA when D is thecalculated diameter of the ultimate particle, d is the density of thesolid inorganic material (e.g., 2.2 grams per cm. for amorphous SiO andA is the specific surface area of the microsphere, as determined bynitrogen adsorption, as disclosed by F. M. Nelson and F. T. Eggersteinin Analytical Chemistry 30, 1387 (1958). At 100 A, the pore diameter isabout equal to the colloidal particle diameter and at about 50A it isabout one and a half times the colloidal particle diameter.

By using colloidal particles, as defined below, the microparticles ofthe present invention will have a diameter in the range of about 50 A toabout 2500 A or more narrowly, about 75 A to about 1000 A.

Microparticles useful as the packing material in the chromatographiccolumn or resolving zone of the present invention can be made by aprocess disclosed in copending US. patent application Ser. No. 242,039for Uniform Oxide Microspheres and a Process for Their Manufacture,which was filed by R. K. Iler and H. .l. McQueston on the same day asthis application. A detailed discussion will be found in thatapplication, the disclosure of which is hereby incorporated by referenceinto this specification, but for convenience, a short explanation of theprocess will be given here. In this process, an aqueous sol of arefractory oxide particle is formed and mixed with a copolymerizablemixture of urea and formaldehyde or melamine and formaldehyde.Polymerization is initiated and coacervation of the organic materialinto microspheres containing the colloidal particles occurs. Themicrospheres are then solidified, collected, washed and dryed. At thisstage, the microspheres consist of a plurality of colloidal particlesembedded in a sphere of polymer. The organic material is then burned offat a temperature sufficient to oxidize the organic constituents withoutmelting the inorganic material. Generally, this is about 550C. Theporous microspheres are then sintered at an elevated temperature for atime sufficient to strengthen the mi-' croparticles to the point wherethey will not fracture in use. A good indication of whether enoughsintering has occurred is when the specific surface area of themicrosphere has been reduced to a value which is at least 10% less thanthe surface area of the colloidal particles themselves.

The formation of the microspheres proceeds by association of theinorganic colloidal particles with the organic coacervate. It ispostulated that the extreme uniformity in both the size of themicrospheres and the distribution of the colloidal particles within themicrosphere depend on an interaction between hydroxyl groups on thesurface of the colloidal particles and portions of the polymer chain.For this reason, at least prior to the onset of polymerization, thecolloidal particles must have hydroxyl groups on their surfaceequivalent to a hydrated oxide surface. The interior of the particlesmay consist of some other material, but the surface must be capable ofbeing hydroxylated. For the purpose of the present invention, theinorganic colloid must also be one that will leave a solid residue afterthe polymer is removed. The preferred colloids, then, are refractorymetal colloids which do not melt or otherwise decompose at about 500C.This is about the lowest temperature that can be used to burn out theorganic material. Generally, however, the refractory particles will havea melting point greater than 1000C., but lower melting oxides can beused if the polymer constituent is removed by slow oxidation at lowertemperatures. Examples of refractory metal oxides that can be used inthe practice of the present invention are: alumina, zirconia, titania,ferric oxide, antimony oxide, tin oxide or combinations thereof, withthe preferred material being silica.

The ultimate particles of the present invention must be colloidal insize. For purposes of the present invention, this means that at leasttwo of the dimensions of these particles must be in the range of 3 to500 millimicrons and the other dimension must be in the range of 3 to1000 millimicrons. Particles having one dimension greater than a micronor having any dimension greater than about 0.1 times the diameter of themicrosphere, are difficult to incorporate into spherical microparticlessince the large dimension interferes with the formation of discretespherical units.

The organic components must be initially soluble in water and misciblewith the inorganic colloid without flocculating or-dissolving it at thepH at which the reaction occurs. The polymer when formed must beinsoluble in water. While a variety of organic materials may besuitable, it appears that the highest degree of uniformity in bothparticle size and pore size distribution occurs when a copolymerizingmixture of urea and formaldehyde or melamine and formaldehyde is used.Urea and formaldehyde in mol ratio of about 1 to about 1.2 or 1.5 and apH of about 1.0 to 4.5, and melamine and formaldehyde in mol ratio ofabout 1 to about 3 and a pH of about 4 to about 6 are suitable.

The ratio of organic material to inorganic material should be such thatafter polymerization, the precipitated particles contains about 10 toabout 90% by weight of the inorganic component. Expressed in terms ofvolume, the percent volume of inorganic material should range from about10 to about 50%. To obtain coherent porous spheres after the organicmatter is burned out, there must be a sufficiently high concentration ofinorganic particles within the matrix to link together into athree-dimensional matrix. This network may be very fragile, whenobtained at 550C, but if heated, undisturbed at higher temperatures, toinitiate sintering, the porous spheres develop strength. To insure thatsufficient sintering has occurred to provide thedesired strength, theparticles are generally sintered at an elevated temperature, usuallyabove 900C, which is sufficiently high to reduce the specific surfacearea of the particle by at least 10% below the value for the colloidalparticles from which they are formed.

The microspheres of the present invention have uniform pores, thediameter of which depends on the size of the colloidal particles used intheir preparation and the volume ratio of the organic polymer to theinorganic material used. The larger the particles, the larger the poresbetween them, and the greater the proportional volume of organic polymerin the microspheres when formed, the more open the network of inorganicparticles and the wider the pores.

in optimum dimensions, these microspheres exhibit superior performancein the various forms of liquid chromatographic applications;liquid-liquid, liquidsolid and exclusion. For example, highly efficientliquid-solid (thin layer and column) chromatography can be carried outwith microspheres having a diameter in the 1.0 to 10.0 micron range madefrom colloidal particles in the 3-100 millimicron range. Very high speedliquid-liquid chromatography can be practicedby coating microsphereshaving a diameter in the 1.0 to 10.0 micron range and made fromcolloidal particles in the to 80 millimicron range, with appropriatestationary liquid phases. These particles may also be reacted with ionexchange media to produce supports for ion exchange chromatography. Theymay be reacted with reagents to produce chromatographic packings withchemically bonded stationary phases. Highly efficient gas-liquid andgas-solid chromatographic separation can be carried out withmicrospheres having a diameter in the range of 50 to l50microns, madefrom colloidal particles in the 50 to 200 millimicron range. The rangeof useful microparticle diameters, therefore, extends from about 0.5 toabout 500 microns.

Since the microspheres prepared from each size colloidal particleconsist of a totally porous structure having a narrow range of poresize, by varying the size'of the colloidal particles, microsphereshaving a predetermined range of relatively homogeneous pore sizes can beproduced. Silica microspheres with pores of known dimension can be usedfor high speed exclusion chromatographic separation (gel permeation andgel filtration) i.e., separation based on the differential migration ofmolecules based on molecular size or molecular weight considerations.The small particle size promotes rapid mass transfer so that carriervelocities much higher than normal can be used while still maintainingequilibrium in the diffusion-controlled interaction which takes placewith the pores in the totally porous structure. The strong, rigidcharacteristics of the microspheres permit the use of very highpressures (at least up to 6000 psi) without particle degradation ordeformation. The spherical nature of the particles permit the packing ofcolumns with large number of theoretical plates, which is of particularimportance in the separation of small molecules. Of prime considerationin the exclusion chromatographic process is the internal volume of theparticles used in the separation. Pore volume is relatively high in themicrospheres, usually from 50-65% (measured by N adsorption by B.E.T.method), depending on pore size, which is comparable to that found forthe porous glasses and the porous organic gels widely used for exclusionchromatography.

It is predicted that the silica microspheres will be useful in gelfiltration separations in aqueous systems and will be particularlyuseful for the separation of small polar molecules. Microspheres havingpores in the 50 to 2500 A range should permit the high-speed exclusionchromatographic separation of a large variety of compounds in bothaqueous and nonaqueous systems.

One of the factors that affects column efficiency is the packing of thecolumn or structure which constitutes the resolving zone. Themicroparticles of the present invention have a distinct advantage inthis respect because their spherical and uniform size contributes to theease with which they can be packed inta dense bed. The most commonpacking practice is dry packing. I have found, however, that the columnperformance can be significantly improved if, during the dry packingprocess, the column is subjected to a vertical motion of controlledfrequency while the dry packing material is fed to the column at aconstant rate.

Dry packing is different when the particles are less than about 20microns in diameter. Another process, high pressure slurry packing, hasbeen used for such particles with some success. I have found that theuniform porous silica microspheres of the present invention can beeasily and conveniently high-pressure slurry-packed into columns afterproducing a stable aqueous suspension. This suspension is accomplishedby ultrasonic mixing of the packing in degassed 0.00l M NH Ol-l. Theadsorbed NHf places a positive charge on each particle, resulting inrepulsion of the particles and stabilization of the slurry with aminimum of aggregation. This approach works particularly well with thetotally porous silica microspheres of the present invention because ofthe uniform particle size. The ammonia-stabilized slurry is rapidlypumped into column blanks at 6000 psi in the usual manner. Water isremoved from the packing by pumping through absolute methanol. Thepacking is then equilibrated with the solvent or solvent/stationaryphase system that is to be employed. For instance, for liquid-solidchromatography, the methanol-treated microspheres are condi- TABLE ISpecific Permeability of High Performance LC Columns Particle K Size, p.Cm X 10 37 2.2

Packing Type Zipax" surface porosity particles Diatomaceous earth,Kieselguhr Silica gel Porous silica microspheres 8-9 Porous silicamicrospheres 5-6 Registered trademark of the El. duPont de Nemours & Co.

The close-sized porous silica microspheres show higher permeability(less resistance to flow) than irregularlyshaped and wider size rangesilica gel and diatomaceous earth particles of the same size. Pressurerequirements for microsphere columns are sufficiently low so as to behandled by most of the pumps currently being used by modern liquidchromatography. One meter microsphere columns of 5-6 p. particles can beoperated at carrier velocities of 0.5 cm./sec. with pressures of onlyabout 2400 psi. Such a column would exhibit 20,000 effective plates,which should permit very difficult separations.

A more meaningful measure of column performance is the PerformanceFactor suggested by L. R. Snyder in Gas Chromatography 1970" publishedby The Institute of Petroleum (page 81). This Performance Factor isequal to (K/(l. 5)"" ]l) when K is the column per ineability, is thecarrier viscosity, and D is calculated from H Dv", where v is thecarrier velocity, H is the plate height, D is a column constant and n isan exponant. Table ll compares the effective plates/sec. and thePerformance Factors for a variety of column packings.

The higher the performance factor, the better the separating ability ofthe column. It can be seen from the table that the porous silicamicrospheres of the present invention compare quite favorably with thepackings of the prior art.

The operation and advantages of the present invention will now be shownby the following examples.

EXAMPLE 1 High-speed liquid-liquid chromatography has been carried outusing the 5-6 p. particles with approximately 350 A pores. Theseparticles, which were made from particles having an average diameter of50 millimicrons, by the process described in Example 1 of the llerapplication discussed above (attorneys docket number lPD-l4) have anitrogen surface area of about 40 m lg. They were selected because ofthe relatively large pores which allows the ready access of solutemolecules to the stationary phase within the totally porous structure. A250 mm X 3.2 mm i.d. stainless steel column was prepared using the highpressure slurry packing procedure discussed above. The packed porousparticles were then filled with B,B'-oxydipropionitrile (BOP) using anin situ technique. In situ coating of column packing may be carried outby any procedure that results in the homogeneous dispersion ofstationary phase throughout the packing. A simple, covenient techniquefor in situ coating of packings, which appears generally applicable, isto pass through the prepacked column the stationary phase dissolved in agood solvent. This step, conveniently carried out at high flow rates, iscontinued until an equilibrium situation exists within the column. Next,a second solvent, which is miscible with the first carrier andimmiscible with the stationary phase, is slowly passed through thecolumn. Under these conditions, the stationary phase is homogeneouslyprecipitated in the internal pores of the support, but is eliminatedfrom the space between the particles when the carrier velocity isincreased. lf high concentrations of the stationary phase in the initialsolvent are used (30-40% by weight) the pores may be completely filledwith the precipitated stationary phase.

This column of silica porous microspheres, contained about 30% by weightof stationary liquid after this treatment. FIG. 3 shows a highefficiency separation of a mixture of hydroxylated aromatic compoundsusing such a microsphere column. The carrier was hexane, at a pressureof 600 psi, a flow of 1.0 ml/min. The separation was carried out at atemperature of 27C. with 4 microliters of solution. The last peak,2-phenylethanol (k' 12), exhibited 6600 theoretical plates (N), or aplate height (H) of 0.038 mm at a carrier velocity of 0.44 cm/sec.Relatively low pressure was needed to operate the column at this flowrate. At a carrier velocity of 3.3 cm/sec. (5000 psi, flow 7.7 cc/min.)this last peak elutes in 73 seconds with a plate height of 0.] l mm, anda N /2 (effective plates/sec) 23.

EXAMPLE 2 High speed liquid-solid chromatographic separations have beencarried out with 8-9 micron silica microspheres, from particles havingan average diameter of 5 millimicrons, made according to Example 1 inthe ller application mentioned above (attorney's docket number lPD-l4).These particles have pores of about 75 A and a nitrogen surface area ofabout 350 m /g. FIG. 4 shows a high efficiency separation of a mixtureof hydroxylated aromatic compounds using a column and conditions similarto that of Example 1 except that the pressure was 2000 psi, temperaturewas 27C., the flow rate was 10.5 ml/min and the carrier fluid wasdichloromethane (half water saturated).

in this separation, seven major compounds were resolved in about 65seconds, the last peak, 3-phenyl-lethanol (capacity factor k E 10)showing the value of 825 effective plates or 14 plates per second N /tat a carrier velocity of 4.7 cm/sec. At a carrier velocity of 9.5cm/sec. (4000 psi) this column exhibited l8 and 24 effective plates/sec,respectively for 3-phenylethanol and benzyhydrol (k' E 4).

EXAMPLE 3 High speed exclusion separation has been carried out using 5-611. particles with approximately 350 A pores. These particles were madefrom small particles having an average diameter of 50 millimicrons, bythe process of the ller application described above (attorneys docketnumber of [PD-l4). FIG. 5 shows three polystyrene fractions (molecularweight 2,030, 51,000, and 41 1,000) separated essentially to base linein about 38 seconds with a 250 mm X2.l mm i.d. column containing 0.5 g.of packing. The carrier fluid was tetrahydrofuran at a temperature of60C., a pressure of 1625 psi and a flow rate of 1.0 ml/min. Such aseparation of polystyrene fractions is unique.

I claim:

1. In an apparatus for use in chromatographic separation comprising aregion through which materials to be separated are passed, theimprovement wherein said region comprises a plurality of uniform sizedporous microspheres having an average diameter of about 0.5 to about 20microns, substantially all of said microspheres having a diameterranging from about 0.5 to about 1.5 times the average diameter of themicrospheres in said powder, said microspheres consisting essentially ofa plurality of uniform-sized colloidal particles, having a refractorymetal oxide surface, arranged in a interconnected three-dimensionallattice, said colloidal particles occupying less than 50% of the volumeof said microspheres with the remaining volume being occupied byinterconnected pores having a uniform pore size distribution.

2. The apparatus of claim 1 in which said microspheres have an averagediameter of about 1.0 to about microns.

3. The apparatus of claim 2 in which substantially all of saidmicrospheres have a diameter ranging from about 0.8 to about 1.2 timesthe average diameter of the microspheres in said powder.

4. The apparatus of claim 2 in which said colloidal particles arecomposed of materials selected from the group consisting of silica,alumina, zirconia, titania, ferric oxide, antimony oxide, tin oxide andcombinations thereof.

5. The apparatus of claim 2 in which said colloidal particles arecomposed of silica.

6. The apparatus of claim 2 in which said colloidal particles occupyless than 50% of the volume of said microspheres.

7. The apparatus of claim 6 in which said microspheres have a specificsurface area greater than about of the surface area of the colloidalparticles from which they are made.

8. The apparatus of claim 2 in which said microspheres are modified witha sorptively active material.

9. In a process for performing chromatographic separation comprisingcontacting the materials to be separated in a carrier phase with aresolving zone, the improvement wherein said resolving zone comprises aplurality of uniform-sized porous microspheres having an averagediameter of about 0.5 to about 20 microns, substantially all of saidmicrospheres having a diameter ranging from about 0.5 to about 1.5 timesthe average diameter of the microspheres in said powder; saidmicrospheres consisting essentially of a plurality of uniform-sizedcolloidal particles, having a refractory metal oxide surface, arrangedin a interconnected threedimensional lattice, said colloidal particlesoccupying less than 50% of the volume of said microspheres with theremaining volume being occupied by interconnected pores having a uniformpore size distribution.

10. The process of claim 9 in which said microspheres have an averagediameter of about 1.0 to about 10 microns.

11. The process of claim 10 in which substantially all of saidmicrospheres have a diameter ranging from about 0.8 to about 1.2 timesthe average diameter of the microspheres in said powder.

12. The process of claim 10 wherein said colloidal particles arecomposed of materials selected from the group consisting of silica,alumina, zirconia, titania, ferric oxide, antimony oxide, tin oxide andcombinations thereof.

13. The process of claim 10 wherein said colloidal particles arecomposed of silica.

14. The process of claim 10 wherein said colloidal particles occupy lessthan 50% of the volume of said microspheres.

15. The process of claim 14 wherein said microspheres have a specificsurface area greater than about 75% of the surface area of the colloidalparticles from which they are made.

16. The process of claim 10 wherein said microspheres are modified witha sorptively active material.

2. The apparatus of claim 1 in which said microspheres have an averagediameter of about 1.0 to about 10 microns.
 3. The apparatus of claim 2in which substantially all of said microspheres have a diameter rangingfrom about 0.8 to about 1.2 times the average diameter of themicrospheres in said powder.
 4. The apparatus of claim 2 in which saidcolloidal particles are composed of materials selected from the groupconsisting of silica, alumina, zirconia, titania, ferric oxide, antimonyoxide, tin oxide and combinations thereof.
 5. The apparatus of claim 2in which said colloidal particles are composed of silica.
 6. Theapparatus of claim 2 in which said colloidal particles occupy less than50% of the volume of said microspheres.
 7. The apparatus of claim 6 inwhich said microspheres have a specific surface area greater than about75% of the surface area of the colloidal particles from which they aremade.
 8. The apparatus of claim 2 in which said micRospheres aremodified with a sorptively active material.
 9. In a process forperforming chromatographic separation comprising contacting thematerials to be separated in a carrier phase with a resolving zone, theimprovement wherein said resolving zone comprises a plurality ofuniform-sized porous microspheres having an average diameter of about0.5 to about 20 microns, substantially all of said microspheres having adiameter ranging from about 0.5 to about 1.5 times the average diameterof the microspheres in said powder; said microspheres consistingessentially of a plurality of uniform-sized colloidal particles, havinga refractory metal oxide surface, arranged in a interconnectedthree-dimensional lattice, said colloidal particles occupying less than50% of the volume of said microspheres with the remaining volume beingoccupied by interconnected pores having a uniform pore sizedistribution.
 10. The process of claim 9 in which said microspheres havean average diameter of about 1.0 to about 10 microns.
 11. The process ofclaim 10 in which substantially all of said microspheres have a diameterranging from about 0.8 to about 1.2 times the average diameter of themicrospheres in said powder.
 12. The process of claim 10 wherein saidcolloidal particles are composed of materials selected from the groupconsisting of silica, alumina, zirconia, titania, ferric oxide, antimonyoxide, tin oxide and combinations thereof.
 13. The process of claim 10wherein said colloidal particles are composed of silica.
 14. The processof claim 10 wherein said colloidal particles occupy less than 50% of thevolume of said microspheres.
 15. The process of claim 14 wherein saidmicrospheres have a specific surface area greater than about 75% of thesurface area of the colloidal particles from which they are made. 16.The process of claim 10 wherein said microspheres are modified with asorptively active material.